It backed up to a fence and a double row of concrete barriers - brushing noisily against a hedge - then two men jumped from the cab into a white Chevrolet Caprice getaway car.

The alarm was raised, but minutes later the truck bomb exploded, killing 19 American servicemen and wounding 400 others. Shredded clothes and mattresses dangled from the torn edges of the structure. The crater, made by one ton of explosives, was four times as deep and three times as wide as the one left by the Oklahoma City bomb.

Warnings of such an attack and anti-American threats had been received by US forces in Saudi Arabia for months, and the bombing at a US training center in Riyadh half a year earlier proved they were serious.

The airmen had long known about the vulnerability of Building 131. The base newsletter just a week before gave a reminder that "everyone must wear their dog tags at all times."

But the hostility in a "friendly" country raises tough questions: So long after the 1991 Gulf War, why are 20,000 US troops still required for the "security" of the Persian Gulf? And despite unprecedented military spending since the war, why can't America's Gulf allies yet protect themselves?

Today the nearly $50-billion-a-year US presence - which Secretary of Defense William Cohen called "a premier example of power projection" during a June visit - is used to enforce Washington's policy of "dual containment" against the two regimes it considers the neighborhood bullies: Iran and Iraq.

But the root of the problem is oil, America's most crucial strategic interest in the Middle East. Because it is the essential lubricant for US and Western economies, access to the 65 percent of the world's known oil reserves in the Gulf ranks as the top priority for US strategists, even higher than close ties with Israel.

"We are in it for as long as needed," says Col. Robert Pollard, commander of US Army forces in Kuwait, echoing the Pentagon's oft-stated policy. "We stuck out Europe during the cold war, we stuck out Korea, and we are committed to protecting our vital interests here."

For years, American planners dreamed of having their own military presence in the Gulf. But despite the US-led victory over Iraq in 1991, fully achieving this aim has meant walking a political and cultural tightrope.

The US has no formal defense treaties here, so a surprised Senate delegation was "just aghast" when told during a January visit that the Pentagon was planning for a 20-to-50-year Gulf deployment.

Most Gulf leaders concede that their security today depends upon the US, though American troops are often viewed suspiciously as Christian "infidels" in the lands of Islam.

For them, any half-century deployment plans will amount to American imperialism. Before the war, barely 1,000 troops were in Saudi Arabia. But since then, the buildup has been aggressive.

"Sovereignty is a very sensitive issue over there," Gen. Binford Peay, commander of US forces in the Middle East, recently told the Senate Armed Services Committee. "They look at any kind of permanent move as being intrusive, burdensome."

But the alternative is "haunting and daunting" for Gulf sheikhs, says John Duke Anthony, head of the National Council on US-Arab Relations. "Iran has three times the population of Iraq, is almost three times the size, and the military of each is greater than all the Gulf states combined."

Egged on by Washington, Gulf states have spent millions on new high-tech weapons systems in recent years. But they have been unable to absorb all the hardware and continue to squabble among themselves, so US forces in the Gulf compensate for their weakness.

"Saudi had been the heart of Arabian Peninsula defense until 1990," says a senior Western diplomat in Doha. "Saudi was 'the protector,' but [the Gulf War] showed that the emperor had no clothes."

After the war, Muslim leaders asked that "atheist" troops never again be relied upon for defense. Saudi Arabia promised to be the "pillar" of security and double its troop strength to 200,000. Neither promise has been kept.

Still, Saudi Arabia has been the largest weapons buyer in the world for a decade. It created an army from scratch in a few decades. But missiles sometimes sit in their boxes, even as more pile up. "The Gulf states are all constrained by the reality of low manpower and can only assimilate so many systems," says retired Lt. Gen. John Yeosock, commander of US, British, and French forces during the Gulf War.

"You don't simply buy a weapons system, put it in the warehouse, and say you have the capability," he says.

Still, tailoring US involvement has been difficult because interests, and perceived threats, vary. Kuwait, for example, is the "front line" against Iraq and worries less about Iran. It is spending $12 billion on an arms program to the year 2004 - equivalent to just one year of Iraq's 1980s spending.

"Kuwait is like a bride, encircled by major powers that have designs on her," says Abdul-Reda Assiri, a political scientist at Kuwait University. "Baghdad has territorial ambitions, the Saudis have political ambitions, and Tehran has ideological ambitions. [This] will continue for the next 60 years."

At the other end of the Gulf, however, in the United Arab Emirates, Iran looms large. In fact, UAE leaders want UN sanctions lifted so they can do business with Iraq.

In the 19th century, Iraq and Iran were seen as balancing powers. The current weakening of Iraq may "encourage" Iran to take advantage of the power vacuum, some say.

US Secretary of State Madeleine Albright said in March that Iraq is now "trapped within a strategic box," though any lack of US resolve "will allow the scorpion that bit us once to bite us again." One Arab official in Abu Dhabi, however, notes that sanctions are hurting the Iraqi people and claims that "Iraq will continue to be weak for 100 years."

That view is scorned by top American officers, who note that Iraq has threatened neighbors several times since the Gulf War.

Part of the problem stems from the Gulf War itself, according to Anthony Cordesman, a senior fellow at the Center for Strategic and International Studies in Washington.

The effectiveness of coalition bombing in the Gulf War was "grossly exaggerated" by the US military, he notes in a recent book on Iraq, so "Iraq remains the leading Gulf power in many areas of force strength."

To complicate the equation, Iran demands that it play an important role. "The problem in the Gulf is the perception that you can buy security," says Javad Zarif, Iran's deputy foreign minister, in an interview. "You can't. The invasion of Kuwait showed us that."

The result has been a mishmash of tenuous American agreements: The Air Force is using remote bases in Saudi Arabia and Turkey to police no-fly zones in Iraq. Saudi refuses to allow port visits by the US Navy or to allow "pre-position" bases.

So the Army has pre-positioned enough hardware for an entire armored brigade in Kuwait and Qatar. US troops rotate through Kuwait, and strains in US-Saudi relations reportedly prompted then-Defense Secretary William Perry last year to call Qatar "the linchpin" in Gulf security. Still, Qatar has rejected dual containment as unworkable.

The Navy was permitted in 1995 to permanently base the headquarters of its recreated 5th Fleet in Bahrain. But tension is high and Arab officials worry that any accident between the US and Iran could spark a crisis.

In such a case, Qatar's emir said during a June visit to Washington, as US allies "we are going to suffer in the Gulf." All of this, an American official explains, because: "It would be embarrassing to lose Kuwait twice."

It took months to deploy US troops before Desert Storm, enough time for Iraq to take control of Saudi's eastern oil fields and to move on Qatar. Today, troops flown from the US can touch down, draw ammunition, and be on the front line facing Iraq in six hours.

"What you are seeing here is the future," a senior Western diplomat says. "Cold-war forces have shrunk, so we must come up with pre-positioning and work ashore and afloat. You 'deep freeze' forces, keep a low profile, and go underground to work on the political arrangements and alliances.

"The problem is that too many people think this is NATO, and that we can do whatever we want," he adds.

Gulf allies also feel pressure from other Arabs, who charge them with selling out to the West. "Kuwait and Saudi Arabia are not really independent states. They are like part of the US," says Iraq's minister of trade, Mahdi Mohammed Saleh, in Baghdad.

In Damascus, Syria, an academic is also disgusted, because Saudi Arabia "only buys weapons to be manned by Americans."

Gulf Arabs admit they can do little alone. "We know the brutal regime in Iraq, and seven years after the invasion we have done nothing to defend our country," says Mohammed al-Qadiri, a former Kuwaiti official.

"When the Iraqi soccer team lost an Asia Cup match recently, and Kuwait was winning, the Iraqis were jubilant. 'We are happy, because our 19th province has won!' they shouted. You can't imagine the feel of those words on our people," he says.